Around the world, what determines the height plants grow to? Some places have trees a hundred meters high, while others are limited to annual plants shorter than a few centimeters. This range of variation is striking, but also important – canopy height helps us predict competition between species, ecosystem carbon and water fluxes, biomass accumulation, and a range of other important processes. Yet the factors that determine plant height have been unclear.
Here are two examples of ecosystems with very different maximum plant height. First, the Sonoran desert, dominated by saguaro cacti (Carnegiea gigantea), no taller than five or ten meters.
Second, an eastern deciduous forest (Michigan), dominated by beech (Fagus grandifolia) and maple (Acer saccharum) trees, with maximum height exceeding twenty or thirty meters.
Note that the Michigan forest is in a much moister environment – is this important? A recent publication has proposed a plausible explanation for these patterns. According to Kempes et al., plant height emerges as an indirect consequence of relationships determining the energy balance and biomass of plants – which in turn depend on precipitation, temperature, and solar radiation. Briefly, they accomplish this by finding the conditions under which the maximum amount of water can flow through a plant of a given size. Read their paper for more details, but known that they are able to make reasonable predictions for tree height across the United States! We are lucky to have a model that can explain such an interesting and easily-observed pattern.